Corrosion- and abrasion-resistant coated metals



Oct. 6, 1959 H. R. SMITH, JR 2,907,679

CORROSION-AND ABRASION-RESISTANT COATED METALS M Filed Oct. 17, 1955 ywdw Attorneys United States Patent CORROSION- AND ABRASION-RESISTANT COATED METALS Hugh R. Smith, Jr., Berkeley, Calif., assignor to Temescal Metallurgical Corporation, Richmond, Califl, a corporation of Californi Application October 17, 1955, Serial No. 540,834 2 Claims (Cl. 117-71) This invention relates to articles of readily deformable materialflexible, malleable or ductileprovided with a surface layer which is resistant to both corrosion and abrasion and which is, moreover, so firmly adherent to the body of the material itself as to retain its continuity, as evident from its corrosionand abrasion-resistance, even when the underlying deformable material isworked as by bending, drawing, or crimping. The invention is particularly applicable to sheet material in large sizes as used for the production of cans and the like, where the sheet must be sharply bent or crimped to form junctions and where, for various reasons, other methods of providing corrosion and abrasion resistant surfaces are uneconomical or impractical.

Illustrative of the kind of problem to which the present invention is a satisfactory solution is the production of fruit containers. The high price of tin and its strategic importance have led to extensive experiments in the attempt to provide a satisfactory substitute for the tin coatings normally applied to the sheet steel which forms the base or substrate from which tin cans are formed. One approach to this problem has been to coat the base sheet with aluminum, which can be done by evaporating a layer of aluminum onto the mild steel base in vacuo. The resultant composite material has many of the qualities desired in a substitute for tin; it presents an attractive appearance, is resistant to many forms (but not all) of corrosion, and it can be prepared in a continuous process. In practice, however, it has proved commercially impracticable. The aluminum coating is soft and sticky and in the manufacture of cans it adheres to and fouls the rollers of the crimping machine so that they have to be retreated or replaced after relatively few operations. r

Means are known for forming corrosionand abrasionresistant surfaces on aluminum. For example, there is the well-known anodizing process whereby the surface to be treated is used as the anode in an electrolytic cell with the production on the surface of a hard and resistant layer of hydrated aluminum oxide, A1 0 The surface formed in this manner is porous, however, and if the porosity is of disadvantage, as it would definitely be in the use of such material to form cans, the pores can be effectively The objects of the present invention are to provide an article of readily deformable material having a surface which is resistant to corrosion, is extremely hard and abrasion resistant, which is, itself, ductile, and so closely adherent to the underlying metal that it effectively maintains continuity when the metal is worked by bending, drawing, crimping or the like; to provide a product which can be manufactured in a continuous process; to provide a sheet material of multi-layer construction wherein the successive layers can be applied to the base in a single operational step; to provide a sheet material wherein the substrate may be of any suitably workable material and which has a hard, corrosionand abrasion-resisting surface, with or without an intermediate layer; and to provide a material which can be manufactured at a cost comparing favorably with the cost of tinplate.

Broadly considered, the invention comprises a base of readily deformable material and a surface layer deposited thereon and firmly adherent thereto having a composition corresponding to a substantially anhydrous oxide of an element of the class consisting of aluminum and silicon and a thickness which is of the order of magnitude of a wavelength of blue light, i.e., a thickness which preferably lies in the range between 5 millionths and 15 millionths of an inch.

Both aluminum and silicon form one oxide which is stable at ordinary room temperatures, these being aluminum trioxide, A1 0 and silicon dioxide, SiO respectively. Under high temperatures both of these oxides vaporize, the temperature of vaporization under atmospheric pressure being in the neighborhood of 2000 C. or somewhat higher. When these oxides are admixed with a reducing agent, however, such as carbon or aluminum, and are heated to a much lower temperature (in the neighborhood of 1300 to 1500 C.) they vaporize in the form of a sub-oxide, A1 0 in the case of aluminum, and SiO in the case of silicon. The compounds. differ in the cases of the different elements but the behaviour in the caseswhere the sub-oxides are formed is substantially identical.

At temperatures under about 1000 C. both of these sub-oxides are unstable. After the sub-oxides strike a cold surface and condense they disproportionate slowly, forming mixed crystals, ALI-A1 0 or Si-i-SiO as the case may be. The resulting material is very hard and very brittle. Silicon oxide or silicon monoxide, formed in this general manner, is a byproduct in certain operations and can be bought under one or the other of these names as an article of commerce. It vaporizes at the same low temperaturre (relative to the vaporization temperature of the dioxides) as in the case of the dioxide in the presence of the reducing agent, but upon solidification again disproportionates, forming a mixture of crystals of the basic element and of the stable oxide. The aluminum sub-oxide behaves in the same general way.

The brittleness of these sub-oxide mixtures is what might naturally be expected from their mixed-crystal character, which can be readily demonstrated by X-ray spectroscopy. In massive form they are as non-malleable as glass or quartz and might be expected to be nonadherent and readily broken up when in the form of a fihn. Silicon oxide layers have been used for the protection of astronomical mirrors and other completely rigid structures, but they would be normally expected to be wholly inappropriate as a protection for metals subjected to abrupt bending or to mechanical working of. any

severe type.

It has been found, however, that when either of these sub-oxides are evaporated in vacuo and condensed of a substrate of sheet-steel (for example), either with or without a previous coating of aluminum the result is a transparent, glass-hard, closely adherent, and apparently elastic layer which retains its continuity under drawing,

3 bending and crimping processes, preventing scratching or corrosion of the substrate.

The products of the present invention, together with the methods of producing them, will be explained more fully in what follows, the products and process being illustrated in the accompanying drawings wherein:

Fig. l is a schematic cross-sectional diagram, on a greatly exaggerated scale, of the two-layer sheet material embodying the present invention;

Fig. 2 is a similar cross-sectional view of a three-layer sheet material; and

Fig. 3 is a schematic illustration of apparatus used in the preparation of the materials illustrated in either Fig. 1 or Fig. 2.

In the form of the invention illustrated in Fig. 1, the base metal 1 can be any of the relatively easily deformable metals which it may be desired to protect. Ordinarily this would be one of the mild steels which can be readily worked; bent, crimped, drawn or otherwise deformed. In the case of material intended to form cans of ordinary size, its thickness may be in the neighborhood of 20 mils or thereabouts. The surface layer 3 of one of the materials listed above is shown greatly exaggerated in proportion to the thickness of the underlying body; preferably it will be in the range between about 5X10- inch and l0- inch, or roughly one one-thousandth that of the sheet which it protects, and its composition corresponds to that of an anhydrous sub-oxide. As stated above, it consists of intimately intermixed crystals of the stable oxide and the base element disproportionated from the sub-oxide.

Fig. 2 is as highly exaggerated as Fig. 1. The inner material to be protected is the layer 5 which may be of steel; on this is imposed an intermediate layer 7 of aluminum which forms the substrate upon which the protective layer 9, of the same nature as the layer 3, is deposited. The thickness of any of the layers except the layer 9 is not of importance as far as the present invention is concerned. Typically, however, the base layer 5 may be 10 to mils in thickness, the layer 7 from 1 to 3 mils, and the layer 9 in the same range as has been described in connection with Fig. 1.

In general, the thickness chosen for the upper, protective layer will be roughly proportioned to the overall thickness of the material. .The general range given covers the cases most usualy'met in practice. The minimum thickness mentioned is enough to resist the abrasion of bending, crimping, and other normal working of the thinner materials and heavy enough to resist scratching. Thicknesses up to something above the maximum thickness begin to show a definite coloration, which in general is not desired. These thicker layers will, however, stand the drawing of the outer surface which accompanies the crimping used in sealing fruit containers, and still retain the continuity which is necessary for protection against corrosion. Corrosion tests of the sub-oxide layer after the material has been bent in a brake indicate this.

' When three layers or more are used (e.g., when both sides are given a protective coating and where an aluminum substrate is used), the material can be worked in crimping machines Without fouling of the rollers and without showing signs of scratching o-r marring due to the treatment that it has received, in spite of the fact that the layer 7 of aluminum, formed in any of the usual ways, is extremely soft.

The preference for the use of a disproportionated suboxide is due to its relatively low cost, and this results from itsmethod of manufacture, the process being illustrated in highly schematic form in Fig. 3. This figure shows a feed roll of sheet material 11, from which a long strip 12 of the base material is drawn by a take-up roll 13: From the feed roll the material passes through a pair of guide rollers 15 into a vacuum system from which it passes through a second set of guide rollers 17 on to the take-up roll.

The vacuum system comprises a series of vacuum chambers 19, 21 into a treatment chamber 23 and thence through a second succession of chambers 21', 19' and out to the guide roll 17. Each chamber is provided with its own separate vacuum pump system of large capacity, represented by the pumps 25, 25, 27, 27, and 29. The pumps are so proportioned that the successive vacuum chambers, between atmospheric pressure and the treatment chamber 23, operate at successively higher vacua, the drop of pressure between atmosphere and the chambers 19, 19', the chambers 19, 19' and 2 1, 21' and the chambers 21, 21 and the treatment chamber 23 are approximately in equal ratio.

Within the treatment chambers are two trough-like boats designated respectively as 31 and 32, which contain, respectively, crucibles 34 and 33. These crucibles are of suflicient width to extend entirely across the strip 12. The boats may be of molybdenum; supplied with current from generators 35 and '37 respectively, they act as resistance elements, heating the crucibles and their contents by conduction and inwardly-directed radiation.

With this arrangement the relatively cold strip passes freely through the entire system at constant speed. As it passes over crucible 34 it receives the substrate of aluminum, the thickness whereof depends upon both the rate at which energy is supplied to the crucible and the speed of the strip. The aluminum condenses immediately upon striking the strip. Passing over the crucible 33 it receives its evaporative coating of sub-oxide, the thickness of the coating again depending upon its rate of speed and upon the rate at which energy is supplied to the crucible.

It will be recognized that one of the great advantages of using coatings of the sub-oxides is the much lower temperature at which they evaporate and the consequent reduction of heat losses, by both conduction and radiation that this entails. At the temperatures required to evaporate any of the oxides the radiation losses form a large part of the total loss and radiation losses vary as the fourth power of the absolute temperatures involved. The power required to supply these losses in the evaporation of the oxides is from two to four times as great as where the sub-oxides are used. Heat insulation problems as well as power problems are therefore simplified by the use of the sub-oxides. Moreover the choice of the materials for both the heating boats and the crucibles is much restricted where higher temperatures must be employed.

Reference has already been made to hydrated oxide coatings, as known in the prior art, typified by those produced by anodizing aluminum, or, in fact, by the coating formed spontaneously on aluminum surfaces. The spontaneously formed coatings are very thin; practically monomolecular, with a thickness nearly an order of magnitude less than that of the coatings here considered, and subject to break-up under working as evidenced by the fouling of crimping rollers mentioned above.

That both the spontaneously formed and anodized coatings must be hydrated is evident not only from the hot-water treatment used to expand and so close the pores of the anodized coating, but from the fact that alumina absorbs water so rapidly that it must be weighed immediately when prepared in the course of gravimetric analysis to prevent false results from absorption of water. This is true of the material when prepared at low temperatures; igniting it to l-1200 converts it to a crystalline form,- as in emery, ruby or sapphire, which are substantially non-hydroscopic. In the coatings formed as here described the disproportionated crystals ofthe oxide appear to be in the anhydrous state and to protect the intimately intermingled crystals of the uncombined elements. Therefore, while it may be that some hydrate is formed where the uncombined crystals are exposed, this appears to occur, if at all, only in a monomolecular layer at the surface, so thin in comparison with the total thickness of the sub-oxide that the description of the layer as a whole as substantially anhydrous is fully justified.

The invention has been described as applied to a steel base-material. This is the application which at present appears to have the greatest utility, but it should be evi-- dent that other base materials which are subject to corrosion or abrasion can be similarly protected, particularly aluminum and its alloys, such as duraluminum.

It is to be understood that while the method of manufacture that has been briefly described is one which has proved suitable in practice, it is not intended to be a limitation upon the scope of the invention as herein set forth, all intended limitations being expressed in the following claims.

What is claimed is as follows:

1. An article of manufacture consisting essentially of a sheet of ductile steel, and a continuous crystalline layer having a composition corresponding to a sub-oxide of an element selected from the group consisting of aluminum and silicon, and a thickness of the order of magnitude of a wavelength of blue light so firmly adherent to the surface of said sheet as to retain its continuity when said sheet is sharply bent.

2. An article of manufacture consisting essentially of a base layer of ductile sheet steel capable of ready deformation, an intermediate layer of aluminum adherent to and deformable with said base layer, and a surface crystalline layer of disproportionated sub-oxide of an element selected from the group consisting of aluminum and silicon and having a thickness in the range of from 5 1.O inch to 15 x 10- inch so firmly adherent to said aluminum layer as to retain its continuity when said base layer is sharply bent.

References Cited in the file of this patent UNITED STATES PATENTS Kafig May 29, 1956 

2. AN ARTICLE OF MANUFACTURE CONSISTING ESSENTIALLY OF A BASE LAYER OF DUCTILE SHEET STEEL CAPABLE OF READY DEFORMATON, AN INTERMEDIATE LAYER OF ALUMINUM ADHERENT TO AND DEFORMABLE WITH SAID BASE LAYER, AND A SURFACE CRYSTALLINE LAYER OF DISPROPORTIONED SUB-OXIDE OF AN ELEMENT SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND SILICON AND HAVING A THICKNESS IN THE RANGE OF FROM 5X10-6 INCH TO 15X10-6 INCH SO FIRMLY ADHERENT OF SAID ALUMINUM LAYER AS TO RETAIN ITS CONTINUITY WHEN SAID BASE LAYER IS SHARPLY BENT. 